This invention relates to a method and apparatus for applying a coating material to a web of paper and more particularly to a coating method and apparatus of the trailing blade type wherein light, heavy or medium weight coatings may be applied in a novel and improved manner.
A conventional coater of the trailing blade type includes means for applying, usually unpressurized, coating material to a paper web that is usually supported and carried by a resilient backing roll, together with a flexible doctor blade located some distance from and on the trailing side of the applicator, which serves to level the applied coating. In general, an excess of coating material is applied to the web, and the trailing blade then meters or removes the excess while uniformly spreading the coating onto the web surface.
In recent years, it has become desirable to produce printed papers having a minimal amount of coating, i.e., in the order of about two or three pounds of coating per ream of paper. When referring to the weight of coating "per ream", it is understood in the art that the term identifies the amount of coating on one side of the paper in the ream. A more complete identification would be "per ream per side".
In order to achieve low coat weights on conventional trailing blade equipment, it is necessary to increase the pressure of the trailing blade against the web, which results in a high rate of wear on the blade and necessitates more frequent replacement of the blade. High blade pressure also increases the possibility of web breakage and streaking caused by foreign particles being caught between the blade and the web.
Many conventional coaters inherently employ a relatively long dwell or soak time, which is the time interval between the initial application and final blading of the coating. As a result, the water portion of the coating composition, as well as the water soluble or dispersible materials contained therein, migrate into the moving web at a more rapid rate than the pigment and eventually cause an undesirable imbalance in the coating constituents and their rheological properties. Long soak periods are also incompatible with the application of successive wet coats without intervening drying because the successive coats tend to migrate into and contaminate the previous coat.
The foregoing problems are discussed in the U.S. Pat. No. 3,348,526 issued to Neubauer wherein a narrow stream of coating is extruded onto an inverted trailing blade that defines a nip region with the supported web. The coating application is such that the coating material is unpressurized after leaving the orifice and supported on the blade or trailing side only, with the leading side of the stream being unsupported and exposed to the environs in the zone of application. Since the coating is bladed immediately after application, soak times are kept to a minimum.